feat: Add user scoped git ssh keys (#834)

coderd/database/migrations/000005_gitsshkey.up.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS gitsshkeys (
+    user_id uuid PRIMARY KEY NOT NULL REFERENCES users (id),
+    created_at timestamptz NOT NULL,
+    updated_at timestamptz NOT NULL,
+    private_key text NOT NULL,
+    public_key text NOT NULL
+);